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  • Is scala functional programming slower than traditional coding?

    - by Fred Haslam
    In one of my first attempts to create functional code, I ran into a performance issue. I started with a common task - multiply the elements of two arrays and sum up the results: var first:Array[Float] ... var second:Array[Float] ... var sum=0f; for(ix<-0 until first.length) sum += first(ix) * second(ix); Here is how I reformed the work: sum = first.zip(second).map{ case (a,b) => a*b }.reduceLeft(_+_) When I benchmarked the two approaches, the second method takes 40 times as long to complete! Why does the second method take so much longer? How can I reform the work to be both speed efficient and use functional programming style?

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  • Scala : reference is ambiguous (imported twice)

    - by tk
    I want to use a method as a parameter of another method of the same class. I have a class and objects which are companions: class mM(var elem:Matrix){ //apply a function on a dimension rows (1) or cols (2) def app(func:Iterable[Double]=>Double)(dim : Int) : Matrix = { ... } //utility function def logsumexp(): Double = {...} } object mM{ def apply(elem:Matrix):mM={new mM(elem)} def logsumexp(elem:Iterable[Double]): Double ={ this.apply(elem.asInstanceOf[Matrix]).logsumexp() } } Normally I use logsumexp like this mM(matrix).logsumexp but if want to apply it to the rows I can't use mM(matrix).app(mM.logsumexp)(1), I get the error: error: reference to mM is ambiguous; it is imported twice in the same scope by import mM and import mM What is the most elegant solution ? Should I change logsumexp() to another class ? Thanks,=)

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  • Why would I use Scala/Lift over Java/Spring?

    - by Chris J
    Hi, I know this question is a bit open but I have been looking at Scala/Lift as an alternative to Java/Spring and I am wonder what are the real advantages that Scala/Lift has over it. From my perspective and experience, Java Annotations and Spring really minimizes the amount of coding that you have to do for an application. Does Scala/Lift improve upon that?

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  • Trimming byte array when converting byte array to string in Java/Scala

    - by prosseek
    Using ByteBuffer, I can convert a string into byte array: val x = ByteBuffer.allocate(10).put("Hello".getBytes()).array() > Array[Byte] = Array(104, 101, 108, 108, 111, 0, 0, 0, 0, 0) When converting the byte array into string, I can use new String(x). However, the string becomes hello?????, and I need to trim down the byte array before converting it into string. How can I do that? I use this code to trim down the zeros, but I wonder if there is simpler way. def byteArrayToString(x: Array[Byte]) = { val loc = x.indexOf(0) if (-1 == loc) new String(x) else if (0 == loc) "" else new String(x.slice(0,loc)) }

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  • Scala Unit type

    - by portoalet
    I use opencsv to parse csv files, and my code is while( (line = reader.readNext()) != null ) { .... } I got a compiler warning saying: comparing values of types Unit and Null using `!=' will always yield true [warn] while( (aLine = reader.readNext()) != null ) { How should I do the while loop?

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  • Best way to score and sum in Scala?

    - by adam77
    Is there a better way of doing this: val totalScore = set.foldLeft(0)( _ + score(_) ) or this: val totalScore = set.map(score(_)).sum I think it's quite a common operation so was expecting something sleeker like: val totalScore = set.sum( score(_) )

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  • Functions without arguments, with unit as argument in scala

    - by scout
    def foo(x:Int, f:Unit=>Int) = println(f()) foo(2, {Unit => 3+4} //case1 def loop:Int = 7 foo(2, loop) //does not compile changing loop to //case 2 def loop():Int = 7 foo(2, loop) // does not compile changing loop to //case 3 def loop(x:Unit): Int = 7 //changing according to Don's Comments foo(2,loop) // compiles and works fine should'nt case 1 and case 2 also work? why are they not working? defining foo as def foo(x:Int, y:()=>Int) then case 2 works but not case 1. Arent they all supposed to work, defining the functions either way. //also i think ()=Int in foo is a bad style, y:=Int does not work, comments??

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  • best scala idiom for find & return

    - by IttayD
    This is something I encounter frequently, but I don't know the elegant way of doing. I have a collection of Foo objects. Foo has a method bar() that may return null or a Bar object. I want to scan the collection, calling each object's bar() method and stop on the first one returning an actual reference and return that reference from the scan. Obviously: foos.find(_.bar != null).bar does the trick, but calls #bar twice.

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  • Get _id from MongoDB using Scala

    - by user2438383
    For a given JSON how do I get the _id to use it as an id for inserting in another JSON? Tried to get the ID as shown below but does not return correct results. private def getModelRunId(): List[String] = { val resultsCursor: List[DBObject] = modelRunResultsCollection.find(MongoDBObject.empty, MongoDBObject(FIELD_ID -> 1)).toList println("resultsCursor >>>>>>>>>>>>>>>>>> " + resultsCursor) resultsCursor.map(x => (Json.parse(x.toString()) \ FIELD_ID).asOpt[String]).flatten } { "_id": ObjectId("5269723bd516ec3a69f3639e"), "modelRunId": ObjectId("5269723ad516ec3a69f3639d"), "results": [ { "ClaimId": "526971f5b5b8b9148404623a", "pricingResult": { "TxId": 0, "ClaimId": "Large_Batch_1", "Errors": [ ], "Disposition": [ { "GroupId": 1, "PriceAmt": 20, "Status": "Priced Successfully", "ReasonCode": 0, "Reason": "RmbModel(PAM_DC_1):ProgramNode(Validation CPG):ServiceGroupNode(Medical Services):RmbTerm(RT)", "PricingMethodologyId": 2, "Lines": [ { "Id": 1 } ] } ] } },

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  • Scala: recursive search avoiding cycles

    - by user1826663
    How can I write a recursive search that I avoid cycles. My class is this: class Component(var name: String, var number: Int, var subComponent: Set[Component]) Now I need a way to check whether a component is contained within its subcomponent or between subcomponent of its subcomponent and so on.Avoiding possible cycles caused by other Component. My method of recursive search must have the following signature, where subC is the Set [component] of comp. def content (comp: Component, subC: Set[Component]) : Boolean = { } Thanks for the help.

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  • scala: how to rewrite this function using for comprehension

    - by opensas
    I have this piece of code with a couple of nasty nested checks... I'm pretty sure it can be rewritten with a nice for comprehension, but I'm a bit confused about how to mix the pattern matching stuff // first tries to find the token in a header: "authorization: ideas_token=xxxxx" // then tries to find the token in the querystring: "ideas_token=xxxxx" private def applicationTokenFromRequest(request: Request[AnyContent]): Option[String] = { val fromHeaders: Option[String] = request.headers.get("authorization") val tokenRegExp = """^\s*ideas_token\s*=\s*(\w+)\s*$""".r val tokenFromHeader: Option[String] = { if (fromHeaders.isDefined) { val header = fromHeaders.get if (tokenRegExp.pattern.matcher(header).matches) { val tokenRegExp(extracted) = header Some(extracted) } else { None } } else { None } } // try to find it in the queryString tokenFromHeader.orElse { request.queryString.get("ideas_token") } } any hint you can give me?

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  • Scalaz: request for use case

    - by oxbow_lakes
    This question isn't meant as flame-bait! As it might be apparent, I've been looking at Scalaz recently. I'm trying to understand why I need some of the functionality that the library provides. Here's something: import scalaz._ import Scalaz._ type NEL[A] = NonEmptyList[A] val NEL = NonEmptyList I put some println statements in my functions to see what was going on (aside: what would I have done if I was trying to avoid side effects like that?). My functions are: val f: NEL[Int] => String = (l: NEL[Int]) => {println("f: " + l); l.toString |+| "X" } val g: NEL[String] => BigInt = (l: NEL[String]) => {println("g: " + l); BigInt(l.map(_.length).sum) } Then I combine them via a cokleisli and pass in a NEL[Int] val k = cokleisli(f) =>= cokleisli(g) println("RES: " + k( NEL(1, 2, 3) )) What does this print? f: NonEmptyList(1, 2, 3) f: NonEmptyList(2, 3) f: NonEmptyList(3) g: NonEmptyList(NonEmptyList(1, 2, 3)X, NonEmptyList(2, 3)X, NonEmptyList(3)X) RES: 57 The RES value is the character count of the (String) elements in the final NEL. Two things occur to me: How could I have known that my NEL was going to be reduced in this manner from the method signatures involved? (I wasn't expecting the result at all) What is the point of this? Can a reasonably simple and easy-to-follow use case be distilled for me? This question is a thinly-veiled plea for some lovely person like retronym to explain how this powerful library actually works.

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  • Macro access to members of object where macro is defined

    - by Marc Grue
    Say I have a trait Foo that I instantiate with an initial value val foo = new Foo(6) // class Foo(i: Int) and I later call a second method that in turn calls myMacro foo.secondMethod(7) // def secondMethod(j: Int) = macro myMacro then, how can myMacro find out what my initial value of i (6) is? I didn't succeed with normal compilation reflection using c.prefix, c.eval(...) etc but instead found a 2-project solution: Project B: object CompilationB { def resultB(x: Int, y: Int) = macro resultB_impl def resultB_impl(c: Context)(x: c.Expr[Int], y: c.Expr[Int]) = c.universe.reify(x.splice * y.splice) } Project A (depends on project B): trait Foo { val i: Int // Pass through `i` to compilation B: def apply(y: Int) = CompilationB.resultB(i, y) } object CompilationA { def makeFoo(x: Int): Foo = macro makeFoo_impl def makeFoo_impl(c: Context)(x: c.Expr[Int]): c.Expr[Foo] = c.universe.reify(new Foo {val i = x.splice}) } We can create a Foo and set the i value either with normal instantiation or with a macro like makeFoo. The second approach allows us to customize a Foo at compile time in the first compilation and then in the second compilation further customize its response to input (i in this case)! In some way we get "meta-meta" capabilities (or "pataphysic"-capabilities ;-) Normally we would need to have foo in scope to introspect i (with for instance c.eval(...)). But by saving the i value inside the Foo object we can access it anytime and we could instantiate Foo anywhere: object Test extends App { import CompilationA._ // Normal instantiation val foo1 = new Foo {val i = 7} val r1 = foo1(6) // Macro instantiation val foo2 = makeFoo(7) val r2 = foo2(6) // "Curried" invocation val r3 = makeFoo(6)(7) println(s"Result 1 2 3: $r1 $r2 $r3") assert((r1, r2, r3) ==(42, 42, 42)) } My question Can I find i inside my example macros without this double compilation hackery?

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  • Scalaz Kleisli question

    - by oxbow_lakes
    There is a trait called Kleisli in the scalaz library. Looking at the code: import scalaz._ import Scalaz._ type StringPair = (String, String) val f: Int => List[String] = (i: Int) => List((i |+| 1).toString, (i |+| 2).toString) val g: String => List[StringPair] = (s: String) => List("X" -> s, s -> "Y") val k = kleisli(f) >=> kleisli(g) //this gives me a function: Int => List[(String, String)] Calling the function k with the value of 2 gives: println( k(2) ) //Prints: List((X,3), (3,Y), (X,4), (4,Y)) My question is: how would I use Scalaz to combine f and g to get a function m such that the output of m(2) would be: val m = //??? some combination of f and g println( m(2) ) //Prints: List((X,3), (X,4), (3,Y), (4,Y)) Is this even possible?

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  • Scalaz: request for use case for Cokleisli composition

    - by oxbow_lakes
    This question isn't meant as flame-bait! As it might be apparent, I've been looking at Scalaz recently. I'm trying to understand why I need some of the functionality that the library provides. Here's something: import scalaz._ import Scalaz._ type NEL[A] = NonEmptyList[A] val NEL = NonEmptyList I put some println statements in my functions to see what was going on (aside: what would I have done if I was trying to avoid side effects like that?). My functions are: val f: NEL[Int] => String = (l: NEL[Int]) => {println("f: " + l); l.toString |+| "X" } val g: NEL[String] => BigInt = (l: NEL[String]) => {println("g: " + l); BigInt(l.map(_.length).sum) } Then I combine them via a cokleisli and pass in a NEL[Int] val k = cokleisli(f) =>= cokleisli(g) println("RES: " + k( NEL(1, 2, 3) )) What does this print? f: NonEmptyList(1, 2, 3) f: NonEmptyList(2, 3) f: NonEmptyList(3) g: NonEmptyList(NonEmptyList(1, 2, 3)X, NonEmptyList(2, 3)X, NonEmptyList(3)X) RES: 57 The RES value is the character count of the (String) elements in the final NEL. Two things occur to me: How could I have known that my NEL was going to be reduced in this manner from the method signatures involved? (I wasn't expecting the result at all) What is the point of this? Can a reasonably simple and easy-to-follow use case be distilled for me? This question is a thinly-veiled plea for some lovely person like retronym to explain how this powerful library actually works.

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  • cannot override a concrete member without a third member that's overridden by both

    - by huynhjl
    What does the following error message mean? cannot override a concrete member without a third member that's overridden by both (this rule is designed to prevent ``accidental overrides''); I was trying to do stackable trait modifications. It's a little bit after the fact since I already have a hierarchy in place and I'm trying to modify the behavior without having to rewrite a lot of code. I have a base class called AbstractProcessor that defines an abstract method sort of like this: class AbstractProcessor { def onPush(i:Info): Unit } I have a couple existing traits, to implement different onPush behaviors. trait Pass1 { def onPush(i:Info): Unit = { ... } } trait Pass2 { def onPush(i:Info): Unit = { ... } } So that allows me to use new AbstractProcessor with Pass1 or new AbstractProcessor with Pass2. Now I would like to do some processing before and after the onPush call in Pass1 and Pass2 while minimizing code changes to AbstractProcessor and Pass1 and Pass2. I thought of creating a trait that does something like this: trait Custom extends AbstractProcessor { abstract override def onPush(i:Info): Unit = { // do stuff before super.onPush(i) // do stuff after } And using it with new AbstractProcessor with Pass1 with Custom and I got that error message.

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  • Accessing a Class Member from a First-Class Function

    - by dbyrne
    I have a case class which takes a list of functions: case class A(q:Double, r:Double, s:Double, l:List[(Double)=>Double]) I have over 20 functions defined. Some of these functions have their own parameters, and some of them also use the q, r, and s values from the case class. Two examples are: def f1(w:Double) = (d:Double) => math.sin(d) * w def f2(w:Double, q:Double) = (d:Double) => d * q * w The problem is that I then need to reference q, r, and s twice when instantiating the case class: A(0.5, 1.0, 2.0, List(f1(3.0), f2(4.0, 0.5))) //0.5 is referenced twice I would like to be able to instantiate the class like this: A(0.5, 1.0, 2.0, List(f1(3.0), f2(4.0))) //f2 already knows about q! What is the best technique to accomplish this? Can I define my functions in a trait that the case class extends? EDIT: The real world application has 7 members, not 3. Only a small number of the functions need access to the members. Most of the functions don't care about them.

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  • Traversable => Java Iterator

    - by Andres
    I have a Traversable, and I want to make it into a Java Iterator. My problem is that I want everything to be lazily done. If I do .toIterator on the traversable, it eagerly produces the result, copies it into a List, and returns an iterator over the List. I'm sure I'm missing something simple here... Here is a small test case that shows what I mean: class Test extends Traversable[String] { def foreach[U](f : (String) => U) { f("1") f("2") f("3") throw new RuntimeException("Not lazy!") } } val a = new Test val iter = a.toIterator

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  • How to split and dispatch an async control-flow using Continuations?

    - by hotzen
    Hello, I have an asynchronous control-flow like the following: ActorA ! DoA(dataA, callback1, callbackOnErrorA) def callback1() = { ... ActorB ! DoB(dataB, callback2, callbackOnErrorB) } def callback2() = { ActorC ! DoC(dataC, callback3, callbackOnErrorC) } ... How would I divide this flow into several parts (continuations) and sequentially dispatch these to different actors (or threads/tasks) while maintaining the overall state? Any hint appreciated, Thanks

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  • Double variable argument list.

    - by Lukasz Lew
    I need something like this: class Node (left : Node*, right : Node*) I understand the ambiguity of this signature. Is there a way around it better than the following? class Node (left : Array[Node, right : Array[Node]) val n = new Node (Array(n1, n2), Array(n3)) Maybe some kind of separator like this? val n = new Node (n1, n2, Sep, n3)

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  • How to find same-value rectangular areas of a given size in a matrix most efficiently?

    - by neo
    My problem is very simple but I haven't found an efficient implementation yet. Suppose there is a matrix A like this: 0 0 0 0 0 0 0 4 4 2 2 2 0 0 4 4 2 2 2 0 0 0 0 2 2 2 1 1 0 0 0 0 0 1 1 Now I want to find all starting positions of rectangular areas in this matrix which have a given size. An area is a subset of A where all numbers are the same. Let's say width=2 and height=3. There are 3 areas which have this size: 2 2 2 2 0 0 2 2 2 2 0 0 2 2 2 2 0 0 The result of the function call would be a list of starting positions (x,y starting with 0) of those areas. List((2,1),(3,1),(5,0)) The following is my current implementation. "Areas" are called "surfaces" here. case class Dimension2D(width: Int, height: Int) case class Position2D(x: Int, y: Int) def findFlatSurfaces(matrix: Array[Array[Int]], surfaceSize: Dimension2D): List[Position2D] = { val matrixWidth = matrix.length val matrixHeight = matrix(0).length var resultPositions: List[Position2D] = Nil for (y <- 0 to matrixHeight - surfaceSize.height) { var x = 0 while (x <= matrixWidth - surfaceSize.width) { val topLeft = matrix(x)(y) val topRight = matrix(x + surfaceSize.width - 1)(y) val bottomLeft = matrix(x)(y + surfaceSize.height - 1) val bottomRight = matrix(x + surfaceSize.width - 1)(y + surfaceSize.height - 1) // investigate further if corners are equal if (topLeft == bottomLeft && topLeft == topRight && topLeft == bottomRight) { breakable { for (sx <- x until x + surfaceSize.width; sy <- y until y + surfaceSize.height) { if (matrix(sx)(sy) != topLeft) { x = if (x == sx) sx + 1 else sx break } } // found one! resultPositions ::= Position2D(x, y) x += 1 } } else if (topRight != bottomRight) { // can skip x a bit as there won't be a valid match in current row in this area x += surfaceSize.width } else { x += 1 } } } return resultPositions } I already tried to include some optimizations in it but I am sure that there are far better solutions. Is there a matlab function existing for it which I could port? I'm also wondering whether this problem has its own name as I didn't exactly know what to google for. Thanks for thinking about it! I'm excited to see your proposals or solutions :)

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  • Cost vs. Fun - Dilemma about learning a serverside language [closed]

    - by Ixx
    I want to learn a new server-side language. I already know Java. I'm fascinated by Scala, have read about it a lot, and want to get practical experience. But I also have some concerns of practical nature - I want to do the backend for many small, non-profit apps, and pay the lowest price for hosting possible. I also want that other people can easily contribute to these apps, if applicable. This leads me in PHP's direction. But I don't like PHP and would like to use these small projects to learn Scala instead. You see the problem, I don't want to pay more money each month, from my own pocket, for hosting. But I want to use these projects to learn Scala. Is there a practical solution for this?

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